The present invention relates to battery backup power systems and, more particularly, to a passive cooling device for extending the life of a battery backup power system used for powering electronic components when utility line power is interrupted or unavailable.
Use of a battery or an electrolytic-type device to provide backup power to electronic equipment during a power outage is known in the art. One such application is with time of use (TOU) watt-hour meters. TOU watt-hour meters permit electric utilities to apply complex rate structures to their customers. TOU watt-hour meters typically sample a customer's energy usage at selected times and record the kilowatts used with the corresponding time of use; thus, different billing rates can be applied for different times of the day, days of the week and even during certain times of the year, such as holidays and different seasons. These meters typically utilize an application specific integrated circuit (ASIC), microprocessor or the like for recording the kilowatts and the time of use. The ASIC will also include a clock for measuring time and maintaining calendar information. The ASIC and clock are powered by the utility line power under normal operating conditions, but these type meters also usually have a battery for backup ASIC power when utility line power is interrupted. The battery will maintain operation of the ASIC clock during power outages to prevent the need for a maintenance visit by utility personnel to reset the system clock and correct for the period of power outage after power is restored.
A lithium-type battery may be used in TOU watt-hour meters to maintain the ASIC clock during power outages, but these type batteries have a limited shelf life and must be replaced about every five years at a cost of about $10 per battery. Maintaining reliable battery backup for TOU watt-hour meters is therefore a considerable burden for utility companies when considering the number of units in the field and the logistics and record-keeping requirements. To meet the needs of most utility companies which use TOU watt-hour meters, a desired backup battery power system will: have a shelf life in excess of about 10 years; and provide approximately 2-5 volts and 1-10 .mu. amps for more than about 30 days. Solid state lithium iodine-type batteries, such as those used in heart pacemakers and the like, typically have a life in excess of about 20 years when operated at body temperature, i.e., about 37.degree. C. However, the life of these pacemaker-type batteries decreases and the current and voltage levels are unreliable when the batteries are stored at higher temperatures. TOU watt-hour meters can be exposed to temperatures as high as about 85.degree. C. for long periods in some climates; under such conditions, the capacity of the battery can decrease by about 80 percent in five years.
Similarly, other types of batteries also experience a decrease in capacity when exposed to high temperatures and therefore it is desirable to maintain battery temperature below a selected level.
Active cooling of a TOU watt-hour meter battery using a thermoelectric (TE) cooling module and associated electronics was disclosed and claimed in co-pending application Ser. No. 07/387,183, filed July 31, 1989, assigned to the same assignee as the present invention, and incorporated herein in its entirety by reference. The TE cooling device and controlling electronics disclosed is highly efficient but has a moderate cost; the utility power line is tapped to provide power to the TE working module when the cooling control circuitry senses a battery temperature in excess of a selected level. Since the TE working module is operated by DC voltage, the AC utility power must be transformed, converted to DC and conditioned to remove harmonics for efficient operation of the TE module. Additionally, the reliability of the TE module, the associated control electronics and the signal transforming and conditioning means is a consideration.
It is accordingly a primary object of the present invention to provide a novel passive cooling device for cooling a temperature-sensitive component.
It is another object of the present invention to provide a novel backup power supply device for a TOU watt-hour meter, which is not subject to the foregoing disadvantages.
It is a further object of the present invention to provide a passive device for cooling the backup battery for a TOU watt-hour meter to extend the useful life of the backup battery.
These and other objects of the invention, together with the features and advantages thereof, will become apparent from the following detailed specification when read with the accompanying drawings in which like reference numerals refer to like elements.